Ultra wideband (UWB) antennas should operate across a bandwidth of at least about 1.5:1 [or 20% at the center frequency, according to the FCC standards] at microwave frequencies; one being from 3.1-10.6 GHz. The antenna should exhibit a reasonably stable input resistance and reactance across the frequency range to accomplish a voltage standing wave ratio (VSWR) lower than two across the antenna's average resistance.
Ultra wideband loop antennas are typically embodied as either a half loop driven against an orthogonal ground plane, or a full loop. Such antennas typically have a feed region that is much narrower than the ground region. Examples are found in U.S. Pat. No. 3,015,101 to Turner et al., U.S. Pat. No. 6,437,756 to Schantz, U.S. Pat. No. 6,914,573 to McCorkle, U.S. Pat. No. 7,132,985 to Lin, and U.S. Pat. No. 7,262,741 to Krupezevic at al. Such antennas are typically modified loops having an elliptical shape with a number of resonances that help to accomplish ultra wideband performance. However, modified loop antennas sometimes have a characteristic impedance that is too high for many applications. Another problem with such antennas is that the impedance may not be stable across the operating range. Also, these antennas can suffer from problematic spatial variation in the radiation pattern as a function of frequency.